1. Field of the Invention
The present invention relates to thermal printers and more particularly to circuitry for supplying energy to thermal print head heat elements.
2. Background of the Invention
As is well known in the art, a thermal print head utilizes a row of closely spaced resistive heat generating elements or thermal point elements which are selectively energized to record data in hard copy form. The data may comprise stored digital information relating to text, bar codes or graphic images. In operation, the thermal print elements receive energy from a power supply through driver circuits in response to the stored digital information. The heat from each energized element may be applied directly to thermal sensitive material or may be applied to a dye-coated web to cause transfer of the dye by diffusion to paper or other receiver material. The Kodak.RTM. XL7700 digital continuous tone printer contains such thermal print elements and operates in this fashion.
The power delivered to the media to form an optical density at a pixel is a function of the power dissipated in the resistive heat generating element. The power dissipated in a thermal print element is equal to the square of the voltage drop across the thermal print element divided by the resistance of the element.
A typical single density image printer is shown functionally in FIG. 1. In the printing mode, an electrical voltage from the power supply, Vs, is applied across the thermal print elements, Re1-Ren. The electronic circuitry to permit current to pass through one or more of the elements exists in the printer and is necessary to perform the printing function. For the purpose of this description, the circuitry can be simplified to a shift register, SR1-SRn, an enable signal, E1, logical gates, AND1-ANDn, and transistor switches, T1-Tn. The complexity of these devices varies for different printers; however, this basic functionality exists in each of the different designs.
In the printing mode, the shift register, SR1-SRn, is loaded with a logical "1" at each location corresponding to a pixel where there is a desire to form an optical density. The outputs of the shift register, SR1-SRn, are logically anded with an enable pulse, E1, in the and gates AND1-ANDn. The enable pulse, E1, is formed to represent the duration that a current is desired to pass through the thermal print elements, Re1-Ren. The output of the gates, AND1-ANDn, biases transistor switches, T1-Tn, to allow current to pass through the corresponding thermal print elements, Re1-Ren, to ground. The energy transferred to the media to form an optical density is typically a function of the voltage drop across the thermal print element and the duration that the current is allowed to pass through the thermal print element.
The relationship of the optical density formed at a pixel to the energy dissipated in the associated thermal print element is calibrated and is expected to remain constant during the time interval between calibrations. If the voltage applied to the thermal print element is changed by some mechanism, the relationship between the optical density formed at a pixel to the power dissipated in the associated thermal print element is also modified. The result of this change is that the optical density formed at the pixel is not predictable or desired. This may be measured as either an increase or decrease in the optical density of the pixel. Thermal print element power dissipation equations that apply in this printer configuration are as follows: ##EQU1##
As shown in Equation 1, the power dissipated in a thermal print element is equal to the square of the voltage drop across the thermal print element, VRe, divided by the resistance of the element, Ret. The voltage VRe is determined by the power supply voltage, Vs, and the voltage divider relationship of the harness resistance and the parallel resistance of the enabled thermal print elements as shown in Equation 2. The number of enabled thermal print elements is signified as n. The power dissipated in a thermal print element, PRet, is equal to the square of the voltage drop across the thermal print element divided by the resistance of the thermal print element, Ret, as shown in Equation 3.
A plot of the power dissipated in one thermal print element versus the number of energized thermal print elements for three possible values of harness resistance is provided in FIG. 3 and Table 1.
TABLE 1 ______________________________________ Power Dissipated in a Thermal Print Element n Rh = 2 Rh = 0.2 Rh = 0.02 ______________________________________ 100 0.191 0.218 0.222 1775 0.040 0.172 0.216 3550 0.016 0.137 0.210 ______________________________________
With a harness resistance Rh of 2.0 ohms, the power dissipated in each of the thermal print elements when 3550 elements are enabled is approximately 10% of the power dissipated when 100 elements are enabled. By reducing the harness resistance Rh to 0.02 ohms, the power dissipated in each of the thermal print elements when 3550 elements are enabled is approximately 95% of the power dissipated when 100 elements are enabled. A 5% power variation dependent upon scene content is an improved condition; however, a 0% variation is desired. This description has not accounted for the effects of resistance variations between thermal print elements and resistance drops in the power distribution bus inside the thermal head, both of which increase the variation in power dissipation of the thermal print elements.
Numerous attempts have been made to automatically correct for resistance variations between thermal print elements and resistance drops in the power distribution bus inside the thermal head which vary over time. Most thermal printers incorporate driver and other circuitry that control print operations so that it is difficult to obtain access to the contacts of individual print head resistive heating elements. It is relatively easy, however, to determine the voltage at the terminals of the print head connectors. But the voltage across the print head includes parasitic drops across power supply lines, interconnections and other wiring internal to the print head. As described above, these parasitic voltage drops are proportional to the number of heat elements turned on for a print line. As a result, the parasitic voltage drops vary considerably as the number of selected heating elements changes. The varying heat element voltage produces noticeable variations in density of the imprinted picture elements or pixals.
Commonly assigned co-pending U.S. patent application Ser. No. 547,353 filed Jul. 2, 1990, incorporated herein by reference in its entirety, addresses these problems and the prior art and proposes solutions which involve the maintenance of a substantially constant voltage across the selected resistive heat elements, independent of the number of selected heat elements in any given printing line. Several other techniques have been proposed to prevent these variations and the density of their resultant print, including employing separate power sources for each of the heating elements forming a thermal head, providing an individual balancing resistor for each of the heating elements in the head, and adjusting the electrical power applied to each of the resistive elements following production of an unacceptable print. U.S. Pat. No. 4,540,991 briefly identifies these prior art approaches and sets forth a further proposal to employ a resistance value variation detector selectively connected to each of the resistive elements in order to derive compensation data based upon resistance variation in the elements. The resistance compensation data is retained in a memory at addresses corresponding to each of the resistive elements in the printing head and the compensation information is read out from the memory to thereby compensate the printing data for each element before that data is applied to the shift register stages of the thermal printing head. A similar technique is disclosed in U.S. Pat. Nos. 4,887,092 and 4,996,487, where the resistance check values are employed diagnostically or employed to indicate the temperature of the resistive element between each printing line. It remains desirable to provide a print head which overcomes the variations in image density from the desired density due to all of the aforementioned factors.
Normally the voltage amplitude or the pulse width of a constant current pulse provided by a single system power supply is modulated with the binary or programmable digital data corrected for temperature and resistance variations of the entire head or the individual printing head elements as described, for example, in U.S. Pat. No. 4,710,783, incorporated herein by reference in its entirety. The operating system depicted in FIGS. 5 and 6 of the '783 patent provide an environment within which the improvements of the present invention may be practiced.